Ligand interactions of the cation-dependent mannose 6-phosphate receptor. Comparison with the cation-independent mannose 6-phosphate receptor

The interactions of the bovine cation-dependent mannose 6-phosphate receptor with monovalent and divalent ligands have been studied by equilibrium dialysis. This receptor appears to be a homodimer or a tetramer. Each mole of receptor monomer bound 1.2 mol of the monovalent ligands, mannose 6-phosphate and pentamannose phosphate with K(d) values of 8 x 10^-6 M and 6 x 10^-6 M, respectively and 0.5 mol of the divalent ligand, a high mannose oligosaccharide with two phosphomonoesters, with a K(d) of 2 x 10^-7 M. When Mn²⁺ was replaced by EDTA in the dialysis buffer, the K(d) for pentamannose phosphate was 2.5 x 10^-5 M. By measuring the affinity of the cation-dependent and cation-independent mannose 6-phosphate receptors for a variety of mannose 6-phosphate analogs, we conclude that the 6-phosphate and the 2-hydroxyl of mannose 6-phosphate each contribute ~ 4-5 kcal/mol of Gibb's free energy to the binding reaction. Neither receptor appears to interact substantially with the anomeric oxygen of mannose 6-phosphate. The receptors differ in that the cation-dependent receptor displays no detectable affinity for N-acetylglucosamine 1'-(α-D-methylmannopyranose 6-monophosphate) whereas this ligand binds to the cation-independent receptor with a poor, but readily measurable K(d) of about 0.1 mM. The spacing of the mannose 6-phosphate-binding sites relative to each other may also differ for the two receptors.